Rearranged structure of an peptide fragment ions: A combination of IRMPD and HDX.

OSCAR HERNANDEZ; BENJAMIN BYTHELL; SYLVÈRE DURAND; MAXIMILIANO ROSSA; BÉLA PAIZS; PHILIPPE MAÎTRE

Vancouver

Conferencia; 60th American Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics.; 2012

American Society for Mass Spectrometry

Novel Aspect IR signature of rearranged an ions; IRMPD spectroscopy on an extended wavelength region and HDX on an integrated mass spectrometer Introduction To improve our understanding of CID of protonated peptides, different MS/MS techniques and theory have been used to characterize the structures of peptide fragments. In this context, IRMPD spectroscopy in the 1000-2000 cm-1 region provided valuable information, for example clear-cut conclusions about oxazolone versus macrocyclic structures of bn ions. Due to spectral congestion in the mid-infrared range, structural assignment for larger fragments can be more controversial and other techniques like Ion Mobility Spectrometry (IMS), gas-phase Hydrogen-Deuterium exchange (HDX), and IR in the 3000-4000 cm-1 region need to be additionally used. We report here our recent investigation of the structure of an and bn fragments using a combination of HDX and IR in the 1000-2000 and 3000-4000 cm-1 regions. Methods All the experiments were performed using a 7 Tesla Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer. IRMPD spectra were recorded in two spectral ranges, namely in the 1000-2000 cm-1 using the IR Free Electron Laser at Orsay (France), and in the X-H (X = C, N, H) stretching region using a tabletop optical parametric oscillator/amplifier (OPO/OPA). In the latter case, IRMPD is assisted using a CO2 laser. For Hydrogen-Deuterium exchange experiments, the stored ions were allowed to react with deuterated agent in the ICR cell at constant background pressure. Molecular Dynamics simulations and quantum chemical calculations were performed on the main structural isomers and protonated forms of the investigated ions to aid interpretation of the IR and HDX raw data. Preliminary Data In the case of the an ions, we were particularly interested in probing the formation of rearranged imine-amide structure [1] which had been suggested for the a4 ion of polyglycine using IRMPD spectroscopy in the 1000-2000 cm-1 region. The structures of a2-a4 ions have been probed using IR spectroscopy in the N-H stretching region, and the results are consistent with those obtained in the mid-infrared. Similarly, in the case of b4 ions with the GGGG, AAAA, and YGGF sequences, the combination of IRMPD in an extended wavelength region (600-2000 and 2800-4000 cm-1) allows for clear conclusion regarding whether macrocyclic ring isomer, resulting from head-to-tail cyclisation, occurs or not. IR spectra in the NH stretching region are less congested, and IR signatures of macrocyclic bn structures can be suggested. Different CID conditions were used to induce peptide fragmentation, including “nozzle-skimmer” dissociation in the source in addition to multiple low-energy collisions with Argon in the linear hexapole of our hybrid FT-ICR. The structure of the resulting an and bn fragments were probed using IRMPD spectroscopy, and no evidence for drastic change of isomer population could be found. This even holds true in the case of the a2 ion of YGGFL which is shown to be generated as two co-existing structures, i.e., the classical cyclic as well as the rearranged imine-amide structure. Quantum chemical calculations show that the formation of the latter from the former is possible because an otherwise high-energy proton transfer is catalyzed by the tyrosine side chain ring. IRMPD spectra of the products of gas phase HDX reactions on an and bn ions shed light on details of the HDX mechanism and combination of HDX kinetics and IRMPD spectroscopy provides a consistent description of structures and dynamics. [1] Bythell, B. J.; Maitre, P.; Paizs, B. J. Amer. Chem. Soc. 2010, 132, 14766-14779.